The k-th Lexicographical String of All Happy Strings of Length n

https://leetcode.com/contest/biweekly-contest-24/problems/the-k-th-lexicographical-string-of-all-happy-strings-of-length-n/

这道题需要反复记忆，反之现场我是想不来，string permutation之类的是薄弱点。

permutation一般都可以用recursion来解决。

class Solution:
    def getHappyString(self, n: int, k: int) -> str:
        def generate(prev):
            if len(prev) == n:
                yield prev
                return 
            
            for ch in "abc":
                if not prev or prev[-1] != ch:
                    for s in generate(prev+ch):
                        yield s
        for i,val in enumerate(generate(""),1):
            if i == k:
                return val
        return ""